1. Field of the Invention
The present invention relates to a method of clamping a substrate on a surface of a substrate support structure, a clamp preparation unit configured to clamp a substrate to a substrate support structure, and a lithography system comprising such a clamp preparation unit.
2. Description of the Related Art
Clamping of a substrate, for example a wafer, onto a surface of a substrate support structure, for example a wafer table, is well known in the semiconductor industry, and in particular in lithography systems. In such lithography systems, the clamped substrate is to be patterned by means of subjection to incident photons or charged particles like ions and/or electrons. The clamping ensures the realization of high precision patterning of a target portion of the substrate surface. Preferably, position control by using clamping is not only used during exposure, but also during handling of the substrate, for example during insertion into and/or removal from the exposure chamber within the lithographic system.
Clamping may be achieved by sucking away air between the substrate and the substrate support structure, i.e. by creating a vacuum in between. However, if aforementioned position control is required in a vacuum environment, such a clamping mechanism will not be effective. Various solutions exist for clamping a substrate in a vacuum environment, for example by means of electromechanical clamping. It will be understood that such a solution is not very suitable for use in combination with one or more beams of charged particles like electrons and/or ions.
International application WO2009/011574 filed by the Applicant, describes a lithography system with a substrate support structure for clamping a substrate by means of a layer of stationary liquid, hereafter referred to as capillary layer. The thickness of the capillary layer is such that between the substrate surface and the surface of the substrate support structure a pressure drop arises. As explained in WO2009/011574, adhesion of the liquid to the surfaces of the substrate on the one hand and the substrate support structure on the other hand causes a circumferentially extending liquid surface, concavely extending between the two surfaces. The so-formed concave liquid surface tends to maintain its shape, even if forces are applied to remove the substrate from the substrate support structure surface.
Under some special circumstances, the substrate clamping mechanism described in WO2009/011574 does not perform in an optimal manner, e.g. due to the presence of voids in the capillary layer. Furthermore, due to evaporation processes, the capillary layer will be present for a limited period of time. As a result, the clamping mechanism may only be useful for a selection of devices to be patterned.